Nobel Lecture: Origin, development, and future of spintronics^*

Figure 1

(Color) Basics of spintronics. (a) Schematic band structure of a ferromagnetic metal showing the energy band spin splitting. (b) Resistivities of the spin up and spin down conduction channels for nickel doped with 1% of several types of impurities (measurements at $4.2\mathrm{K}$) (20, 21, 22). The ratio $\alpha$ between the resistivities ${\rho }_{0\downarrow }$ and ${\rho }_{0\uparrow }$ of the spin ↓ and spin ↑ channels can be as large as 20 (Co impurities) or, as well, smaller than 1 (Cr or V impurities). (c) Schematic for spin-dependent conduction through independent spin ↓ and spin ↑ channels in the limit of negligible spin mixing [${\rho }_{\uparrow \downarrow }=0$ in the formalism of 20, 21, 22].

Figure 2

(Color) Experiments on ternary alloys based on the same concept as that of the GMR (20, 21, 22). (a) Schematic for the spin-dependent conduction in alloys doped with impurities of opposite scattering spin asymmetries (${\alpha }_{\mathrm{A}}={\rho }_{\mathrm{A}\downarrow }∕{\rho }_{\mathrm{A}\uparrow }>1$, ${\alpha }_{\mathrm{B}}={\rho }_{\mathrm{B}\downarrow }∕{\rho }_{\mathrm{B}\uparrow }<1$, ${\rho }_{\mathrm{AB}}⪢{\rho }_{\mathrm{A}}+{\rho }_{\mathrm{B}}$) and experimental results for $\mathrm{Ni}\left({\mathrm{Co}}_{1-x}{\mathrm{Rh}}_x\right)$ alloys of total concentration 1%. (b) Same for alloys doped with impurities of similar scattering spin asymmetries (${\alpha }_{\mathrm{A}}={\rho }_{\mathrm{A}\downarrow }∕{\rho }_{\mathrm{A}\uparrow }>1$, ${\alpha }_{\mathrm{B}}={\rho }_{\mathrm{B}\downarrow }∕{\rho }_{\mathrm{B}\uparrow }>1$, ${\rho }_{\mathrm{AB}}\approx {\rho }_{\mathrm{A}}+{\rho }_{\mathrm{B}}$) and experimental results for $\mathrm{Ni}\left({\mathrm{Au}}_{1-x}{\mathrm{Co}}_x\right)$ alloys of total concentration 1%. In GMR the impurities A and B are replaced by magnetic layers, the situation of (a) [(b)] corresponding to the antiparallel [parallel] magnetic configurations of adjacent magnetic layers.

Figure 3

(Color) First observations of giant magnetoresistance. (a) $\mathrm{Fe}∕\mathrm{Cr}\left(001\right)$ multilayers (2) [with the current definition of the magnetoresistance ratio $\mathrm{MR}=100(R_{\mathrm{AP}}-R_{\mathrm{P}})∕R_{\mathrm{p}}$, $\mathrm{MR}=85\%$ for the $(\mathrm{Fe}3\mathrm{nm}∕\mathrm{Cr}0.9\mathrm{nm})$ multilayer]. (b) $\mathrm{Fe}∕\mathrm{Cr}∕\mathrm{Fe}$ trilayers (6). (c) Schematic of the mechanism of the GMR. In the parallel magnetic configuration (bottom), the electrons of one of the spin directions can go easily through all the magnetic layers and the short circuit through this channel leads to a small resistance. In the antiparallel configuration (top), the electrons of each channel are slowed down every second magnetic layer and the resistance is high. From 13.

Figure 4

(Color) (a) Variation of the GMR ratio of $\mathrm{Co}∕\mathrm{Cu}$ multilayers in the conventional current in plane (CIP) geometry as a function of the thickness of the Cu layers (54). The scaling length of the variation is the mean-free path (short). (b) Structure of multilayered nanowires used for CPP-GMR measurements. (c) CPP-GMR curves at 77 K for (Permalloy $12\mathrm{nm}$/copper $4\mathrm{nm}$) multilayered nanowires (solid lines) and (cobalt $10\mathrm{nm}$/copper $5\mathrm{nm}$) multilayered nanowires (dotted lines) (61; 26). (d) Variation of the CPP-GMR ratio of $\mathrm{Co}∕\mathrm{Cu}$ multilayered nanowires as a function of the thickness of the Co layers (61; 26). The scaling length of the variation is the spin diffusion length (long). The inset shows the curves for Permally/copper at 4.2 K.

Figure 5

(Color) GMR head for hard disk. From 13.

Figure 6

(Color) Schematic representation of the spin accumulation at an interface between a ferromagnetic metal and a nonmagnetic layer. (a) Spin up and spin down currents far from an interface between ferromagnetic and nonmagnetic conductors (outside the spin-accumulation zone). (b) Splitting of the chemical potentials $E_{\mathrm{F}\uparrow }$ and $E_{\mathrm{F}\downarrow }$ at the interface. The arrows symbolize the spin flips induced by the spin-split out of equilibrium distribution. These spin flips control the progressive depolarization of the electron current between the left and the right. With an opposite direction of the current, there is an inversion of the spin accumulation and opposite spin flips, which polarizes the current when it goes through the spin-accumulation zone. (c) Variation of the current spin polarization when there is an approximate balance between the spin flips on both sides (metal/metal) and when the spin flips on the left side are predominant (metal/semiconductor without spin-dependent interface resistance, for example). From 13.

Figure 7

(Color) (a) Principle of the magnetic random access memory (MRAM) in the basic “cross point” architecture. The binary information “0” and “1” is recorded on the two opposite orientations of the magnetization of the free layer of magnetic tunnel junctions (MTJs), which are connected to the crossing points of two perpendicular arrays of parallel conducting lines. For writing, current pulses are sent through one line of each array, and only at the crossing point of these lines the resulting magnetic field is high enough to orient the magnetization of the free layer. For reading, one measures the resistance between the two lines connecting the addressed cell. Schematic from 13. (b) High magnetoresistance, $\mathrm{TMR}=(R_{\max }-R_{\min })∕R_{\min }$, measured by 44 for the magnetic stack: ${\left({\mathrm{Co}}_{25}{\mathrm{Fe}}_{75}\right)}_{80}{\mathrm{B}}_{20}\left(4\mathrm{nm}\right)∕\mathrm{Mg}\mathrm{O}\left(2.1\mathrm{nm}\right)∕{\left({\mathrm{Co}}_{25}{\mathrm{Fe}}_{75}\right)}_{80}{\mathrm{B}}_{20}\left(4.3\mathrm{nm}\right)$ annealed at $475°\mathrm{C}$ after growth, measured at room temperature (closed circles) and low temperature (open circles).

Figure 8

(Color) Physics of TMR illustrated by the decay of evanescent electronic waves of different symmetries in a MgO(001) layer between cobalt electrodes calculated by 80. The ${\Delta }_1$ symmetry of the slowly decaying tunneling channel is well represented at the Fermi level of the spin conduction band of cobalt for the majority spin direction and not for the minority spin one, so that a good connection by tunneling between the electrodes exists only for the parallel magnetic configuration when a ${\Delta }_1$ channel can be connected to both electrodes (above). In the antiparallel configuration (below), both the spin up and spin down ${\Delta }_1$ channels are poorly connected on one of the sides. This explains the very high TMR of this type of junction.

Figure 9

(Color) Four state resistance of a tunnel junction composed of a biferroic tunnel barrier $\left({\mathrm{La}}_{0.1}{\mathrm{Bi}}_{0.9}\mathrm{Mn}{\mathrm{O}}_3\right)$ between a ferromagnetic electrode of ${\mathrm{La}}_{2∕3}{\mathrm{Sr}}_{1∕3}\mathrm{Mn}{\mathrm{O}}_3$ and a nonmagnetic electrode of gold. The states 1–4 correspond to the magnetic (white arrows) and electric (black arrows) polarizations represented on the right side of the figure. From 28.

Figure 10

(Color) Illustration of the spin-transfer concept introduced by 69. A spin-polarized current is prepared by a first magnetic layer F with an obliquely oriented spin polarization with respect to the magnetization axis of a second layer ${\mathrm{F}}_2$. When this current goes through ${\mathrm{F}}_2$, the exchange interaction aligns its spin polarization along the magnetization axis. As the exchange interaction is spin conserving, the transverse spin polarization lost by the current has been transferred to the total spin of ${\mathrm{F}}_2$, which can also be described by a spin-transfer torque acting on ${\mathrm{F}}_2$. This can lead to a magnetic switching of the ${\mathrm{F}}_2$ layer or, depending on the experimental conditions, to magnetic oscillations in the microwave frequency range. From 13.

Figure 11

(Color) Experiments of magnetic switching and microwave generation induced by spin transfer from an electrical dc current in trilayered magnetic pillars. (a) Schematic of a trilayered magnetic pillar. (b) Switching by spin transfer between the parallel and antiparallel magnetic configurations of a $\mathrm{Co}∕\mathrm{Cu}∕\mathrm{Co}$ metallic pillar (31; 29). The switching between parallel and antiparallel orientations of the magnetizations of the two magnetic layers of the trilayer is detected by irreversible jumps of the resistance at a critical value of the current. The critical current density is of the order of ${10}^7\mathrm{A}∕{\mathrm{cm}}^2$. (c) Switching by spin transfer of a pillar-shaped tunnel junction composed of electrodes of the dilute ferromagnetic semiconductor GaMnAs separated by a tunnel barrier of InGaAs (19). The critical current is about 100 times smaller than in the Py/Cu/Py pillar. Similar results have been obtained by 33. (d) Typical microwave power spectrum of a $\mathrm{Co}∕\mathrm{Cu}∕\mathrm{Py}$ pillar $(\mathrm{Py}=\text{Permalloy})$ (7; 8).

Figure 12

(Color) Spintronics with semiconductors illustrated by experimental results (49; 24) on the structure represented on the right side and composed of a GaAs layer separated from the GaMnAs source and drain by tunnel barriers of AlAs. (a) MR curve at $4.2\mathrm{K}$ showing a resistance difference of 40% between the parallel and antiparallel magnetic configurations of the source and the drain. (b) MR ratio as a function of the resistance of the tunnel barriers.

Figure 13

(Color) Spintronics with molecules as illustrated. Left: Artistic view of spin transport through a carbon nanotube between magnetic electrodes (courtesy of T. Kontos). Right: Magnetoresistance experimental results (34) at $4.2\mathrm{K}$ on carbon nanotubes between electrodes made of the ferromagnetic metallic oxide ${\mathrm{La}}_{2∕3}{\mathrm{Sr}}_{1∕3}\mathrm{Mn}{\mathrm{O}}_3$. A contrast of 72% and 60% is obtained between the resistances for the parallel (high field) and antiparallel (peaks) magnetic configurations of the source and drain. The voltage difference at constant current can reach 60 mV.